On gradiometer imbalance

We present methods to compute the imbalance in a gradiometer of arbitrary shape due to imperfections in its geometry, eddy currents induced in the radio-frequency interference shield, and screening currents induced in the modules of the superconducting quantum interference devices (SQUIDs). As an example, the methods are applied to evaluate the maximum expected initial imbalance of second- and third-order axial gradiometers in a measuring setup designed for fetal magnetocardiography. Mechanical imperfections in this specific setup appear to have the largest effect: the field imbalance is 2middot10 -2 ; the first-order gradient imbalance is 10 -3 m; the second-order gradient imbalance is 10 -4 m 2 . In the example, the imbalance caused by the other effects is one order smaller.